Polyatomic molecules methane molecule

We will also discuss first progress in the quantum dynamical treatment of polyatomic molecules (methane) reacting on surfaces, which has come possible, thanks to the development of reduced dimensionality models treating at least one vibrational mode of the molecule, and modeling the surface motion. 

Other flexible framework calculations of methane diffusion in silicalite have been performed by Catlow et al. (64, 66). A more rigorous potential was used to simulate the motion of the zeolite lattice, developed by Vessal et al. (78), whose parameters were derived by fitting to reproduce the static structural and elastic properties of a-quartz. The guest molecule interactions were taken from the work of Kiselev et al. (79), with methane treated as a flexible polyatomic molecule. Concentrations of 1 and 2 methane molecules per 2 unit cells were considered. Simulations were done with a time step of 1 fs and ran for 120 ps. 

Valence-bond theory, as described so far, cannot account for bonding in polyatomic molecules like methane, CH4, nor for their bond angles. For example, if we tried to apply the theory to methane, we would note that... 

How does valence bond theory describe the electronic structure of a polyatomic molecule, and how does it account for molecular shape Let s look, for example, at a simple tetrahedral molecule such as methane, CH4. There are several problems to be dealt with. 

R. S. Mulliken, /. Chem. Phys., 3, 517 (1935). Electronic Structures of Polyatomic Molecules. IX. Methane, Ethane, Ethylene, Acetylene. 

A key question about the use of any molecular theory or computer simulation is whether the intermolecular potential model is sufficiently accurate for the particular application of interest. For such simple fluids as argon or methane, we have accurate pair potentials with which we can calculate a wide variety of physical properties with good accuracy. For more complex polyatomic molecules, two approaches exist. The first is a full ab initio molecular orbital calculation based on a solution to the Schrddinger equation, and the second is the semiempirical method, in which a combination of approximate quantum mechanical results and experimental data (second virial coefficients, scattering, transport coefficients, solid properties, etc.) is used to arrive at an approximate and simple expression. 

Among the first polyatomic molecules in which rotational relaxation was investigated is methane. Kelly [278], employing an ultrasonic interferometer, concluded that for this gas as 314°K, Zr = Mtt 15 collisions. More recent ultrasonic data obtained by Hill and Winter [279] provide apparent rotational relaxation times for CH4 and C2H4 as a function of temperature. Their results are given in Table 3.5. The increase in Zr with increasing temperature... 

Usually, the processes are stopped by addition of a quench gas to the main filling gas. Vapours of polyatomic molecules such as ethanol, ether, ethyl formate, methane, bromine or chlorine may be applied. Because of the lower ionization energy of these molecules, the positive charge of the ions is transferred to the molecules and these dissipate their energy by dissociation or predissociation. Chlorine and bromine exhibit strong absorption of the photons emitted they dissociate, recombine and return to the ground state via a series of low-energy excited states. 

This analysis may be extended to formally achiral molecules that are composed of four or more atoms. The motions in such polyatomic molecules are restricted by the restoring forces imposed by bonding, and stochastic achirality is here the result of internal vibrations. Thus, for example, molecular deformations in some vibrational states impart chirality to the methane molecule, but the sense of chirality averages to zero under the conditions of measurement. As this discussion makes clear, the conventional symmetry of methane is a property solely of the model. 

For polyatomic molecules such as methane, the Lennard-Jones equation may be written as... 

Some simple covalent polyatomic molecules are CH4, NH3, OH2 and HF. In the case of methane ... 

Methane and carbon tetrachloride represent the simplest type of covalent polyatomic molecule, involving an valence shell configuration of eight electrons, four shared electron pairs and four covalent bonds and only differing in the terminal atoms. H has a shared configuration and Cl has an configuration, with only one of its four electron pairs shared with the carbon atom. 

We have confined ourselves to spherically symmetric atoms. It is interesting to note that polyatomic molecules with spherically symmetric polarizability (e.g. methane, CH4 and carbon tetra-fiuoride, CF4) can be treated in the same manner. 

Methane was the first polyatomic molecule other than CO2 for which vibrational energy-transfer pathways and rates were investigated. This study of methane involved the utilization of the phase-shift method for determining the lifetimes of vibrationally excited states. The asymmetric stretching vibration, v-, of methane at 3010 cm (see Fig. 1) was excited by a chopped He-Ne laser operating on the 2947.9 cm Ne transition. Fluorescence was detected from both the mode and the bending mode of methane at 1306 cm . Rates were extracted from phase-shift measurements, and Ref. 16 provides an excellent discussion of the background for, and use of the phase-shift method. 

Because only a few nonlinear molecules—methane, ozone, and cyclopropane—have been studied to date, and because no studies of electron impact excitation of such molecules have yet been made, it is too early to assess the utility of current implementations of the / -matrix method for the study of general polyatomic molecules. In particular, it remains to be seen whether, in addition to the usual scaling problems that render ab initio calculations by all methods rapidly more expensive as the number of heavy atoms increases, there are complications peculiar to the the /f-matrix method or its implementation that will arise when the molecular symmetry is low and the number of heavy atoms large. 

The kinetics of dissociation of polyatomic molecules, in particular CH4, stimulated by vibrational excitation in conditions of not very high non-equilibrium parameters y = (Tv — To)/To has been analyzed by Kuznetsov (1971). The CH4 dissociation (9-13) proceeds through vibrational excitation of CH4 molecules at any parameters y = (Tv - To)/Tq. The vibrational energy distribution is an essentially non-Boltzmann distribution in this case, and it is characterized by both temperatures, Tv and To, even when Tv > Tq. The rate coefficient kR(To, Tv) of the methane dissociation (9-13) in non-equihbrium conditions (Tv > To) can be expressed as follows (Kuznetsov, 1971) ... 

It is relatively simple to assign bond enthalpies to the bond in a diatomic molecule because in these cases the bond enthalpy is just the energy required to break the molecule into its atoms. However, many important bonds, such as the C—H bond, exist only in polyatomic molecules. For these bonds, we usually use average bond enthalpies. For example, the enthalpy change for the following process in which a methane molecule is decomposed into its five atoms (a process called atomization) can be used to define an average bond enthalpy for the C—H bond, H(C—H) ... 

S.-Y. Liu and C. E. Dykstra, Chem. Phys. Lett., 119, 407 (1985). Polarizabilities and Hy-peipolarizabilities of Methane. The Importance of Valence Charge Polarization in Polyatomic Molecules. 

A substance composed of polyatomic molecules with approximately spherical shapes might be expected to crystallize in an approximately close-packed structure. Methane, CH4, is an example. Solid methane has a face-centered cubic lattice with... 

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